In recent years, as portable electronic equipment such as video cameras and notebook type computers has widely been used, the demand for small high-capacity secondary batteries has increased as power sources therefor. Most of the secondary batteries now being used are nickel-cadmium batteries containing an alkali electrolyte, the battery voltage thereof being about 1.2 V. For this reason, a nickel-hydrogen battery has received attention as a higher-power battery, and also a lithium battery has been developed.
The nickel-hydrogen battery works with hydrogen used as an active material for negative electrode. The negative electrode thereof is formed by supporting a hydrogen occlusion alloy capable of reversibly occluding/discharging hydrogen on the collector, and the positive electrode is formed by similarly supporting, for example, nickel hydroxide, which is an active material for positive electrode, on the collector.
For example, when a negative electrode of nickel-hydrogen battery is manufactured, predetermined amounts of hydrogen occlusion alloy powder, conductive material powder such as nickel, and binder powder such as polyvinylidene fluoride are mixed to yield a mixed powder, to which, for example, carboxymethyl cellulose solution is added, by which a slurry, which is a mixture for the negative electrode, is prepared. A collector such as a punching Ni sheet with a desirable opening ratio, a Ni foam sheet with a desirable porosity, or a Ni powder sintered body is filled with the slurry. The mixture is supported on the surface of the collector and in the inside voids thereof in a contacting state by sequentially performing drying, rolling, and heat treatment.
When a positive electrode is manufactured, predetermined amounts of nickel hydroxide powder, which is an active material for the positive electrode, and a conductive material such as nickel powder are mixed to yield a mixed powder, to which, a predetermined amount of, for example, carboxymethyl cellulose solution is added, and the whole mixture is agitated into a slurry form, by which a mixture for the positive electrode is prepared. Thereafter, a collector such as a Ni foam sheet is filled with the mixture for the positive electrode. The mixture for the positive electrode is supported on the collector by sequentially drying and rolling it.
Lithium batteries are broadly classified into metallic lithium batteries and lithium ion batteries.
For the metallic lithium battery, the negative electrode is formed of metallic lithium, and the positive electrode is formed by supporting an active material for positive electrode such as LiCoO.sub.2 on a collector. For the lithium ion battery, the positive electrode is formed in the same manner as described above, but the negative electrode is formed by supporting, for example, carbon (C) capable of occluding/discharging lithium ions on a collector.
In the case of the former battery of the batteries of the two types, dendrite recrystallized lithium is deposited on the surface of metallic lithium, which is the negative electrode, during charging, and it grows as the charging/discharging cycle proceeds, so that the battery cycle life is decreased. In the worst case, the grown recrystallized lithium breaks a separator interposed between the positive and negative electrodes, sometimes causing a short circuit.
Thereupon, regarding the lithium battery, the research and development of a lithium ion battery incorporating a negative electrode formed by supporting carbon on the collector is now being carried on. This negative electrode does not present the problem with metallic lithium negative electrode during the charging/discharging cycle.
When a positive electrode of a lithium battery is manufactured, predetermined amounts of, for example, LiCoO.sub.2 powder, which is an active material for the positive electrode, for example, C powder, which is a conductive material, and, for example, polyvinylidene fluoride, which is a binder, are first mixed to yield a mixed powder, to which a predetermined amount of nonaqueous solvent such as N-methylpyrrolidone is added. The whole mixture is mixed thoroughly, by which a pasted mixture, which is a mixture for the positive electrode, is prepared. Then, the mixture is applied onto the surface of collector consisting of metal foil or alloy foil such as Ni, Cu and Ti--Al foil made by, for example, rolling. Thereafter, the mixture for the positive electrode is dried to be put on the collector so as to be in firm contact and integral with the collector.
When a negative electrode of a lithium ion battery is manufactured, fiber-form, woven cloth-form, or felt-form carbon fiber itself is sometimes used as C. In general, however, predetermined amounts of C powder, the aforementioned binder powder, and nonaqueous solvent are mixed to prepare a pasted mixture for the negative electrode, and the mixture is applied to the collector consisting of a metal foil and pressed on it after being dried.
An important point for the aforementioned the positive and negative electrodes is that the mixture for positive or negative electrode (hereinafter called the electrode mixture) does not peel off from the collector when the electrode is incorporated into a battery or at the time of a charging/discharging cycle. If the mixture peels off from the collector, polarization begins to increase in the process of charging/discharging cycle, which causes the cycle life characteristics to decrease.
When a Ni foam sheet is used as a collector as in the case of the hydrogen-nickel battery, the electrode mixture is less prone to peel off because it fills the inside of the sheet.
However, the pore diameter of such a foam sheet, which is about 100 .mu.m, is too large with respect to the whole sheet. Therefore, although this pore diameter is preferable from the viewpoint of increased filling amount of electrode mixture and useful to prevent the electrode mixture from peeling off, it decreases the mechanical strength of the sheet, so that the sheet is prone to be broken. Also, the filling of electrode mixture is nonuniform, so that the electron transfer reaction in the charging/discharging cycle is prone to be nonuniform.
When a punching metal sheet, in which openings of a predetermined diameter are formed regularly, for example, in a zigzag lattice pattern, is used as a collector, the opening diameter is too large with respect to the whole sheet as in the case of a foam sheet, and in manufacturing, an opening-less sheet must be punched, resulting in an increase in cost.
Sometimes, an expanded metal is used as a collector. To manufacture the expanded metal, a nonporous sheet must be subjected to special fabrication as in the case of the punching metal sheet, so that the cost of expanded metal is higher than that of the punching metal sheet.
In the case of the positive or negative electrode for a lithium battery, as described above, a metal foil usually manufactured by rolling is used as the collector, and paste such as an electrode mixture is simply applied to and pressed on the smooth surface thereof, so that peeling occurs easily.
For an electrode in which an electrode mixture is supported on both surfaces of the collector, it is very difficult to apply paste in the completely same thickness on both surfaces. The collector used is generally a rolled nonporous foil, so that lithium ions cannot migrate from one surface of the collector to the other surface thereof.
Therefore, during the charging/discharging, it is impossible to completely use the electrode mixture supported Ion both surfaces of the collector.
An object of the present invention is to provide a method of manufacturing a porous electrolytic metal foil, whereby in the process of making a metal foil by electrolytic plating, a porous metal foil structure can be formed simultaneously with the progress of foil making.
Another object of the present invention is to provide a method of manufacturing a porous electrolytic metal foil, in which a porous electrolytic metal foil, which is useful as a collector for a secondary battery electrode, is manufactured continuously and in large quantities, and therefore at a low cost.